Philips Technical Review, vol. 41, no. 11/12, 1983/84, pp. 348-361, discloses a device of the kind described in the opening paragraph in which a shaft is supported by five pairs of electromagnets in such a way that rotation of the shaft is possible only about the centerline of the shaft. A force exerted on the shaft by an electromagnet is substantially directly proportional to the square of the value of the current through the electromagnet and substantially inversely proportional to the square of the size of an air gap between the electromagnet and the shaft. As a result of the relation between the electromagnetic force and the size of the air gap, the position of the shaft relative to the electromagnets is not stable unless additional measures are taken. To maintain a stable desired position of the shaft in the known device, the position of the shaft relative to each pair of electromagnets is measured by means of a position sensor, and a control current determined from the difference between the measured and the desired position is passed through the two electromagnets of the relevant pair. The value of the control current is thereby determined by means of a control unit having a proportional, differentiating and integrating action PID controller.
The force exerted on the shaft by an electromagnet is always an attracting force, irrespective of the direction of the current through the electromagnet. It is necessary for the electromagnetic forces of the two electromagnets of each pair to be different in value if a support of the shaft is to be obtained by means of the electromagnets. In the known device, an identical basic current is passed through the two electromagnets of each pair for this purpose. The control current is added to the basic current in one of the two electromagnets and subtracted from the basic current in the other electromagnet, so that the forces exerted by the relevant electromagnets are different in value in the case of a change in gap size caused by a loading force. The use of a basic current means that electrical resistance losses arise in the electromagnets caused by this basic current, which leads to heat generation in the electromagnets. This is not acceptable in all circumstances.